The present invention relates to the art of communications, particularly wireless communications.
In telecommunications and computer networks, a channel access method or multiple access method allows several users or terminals connected to the same channel or transmission medium to transmit over it and to share its capacity. Examples of shared physical media are wireless networks, bus networks, ring networks, star networks and half-duplex point-to-point links.
A channel-access scheme is based on a multiplexing method that allows several data streams or signals to share the same communication channel or physical medium. In the art, four basic methods for channel multiple access are known: Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), Space Division Multiple Access (SDMA). In practice, many systems apply a combination of several of these basic multiple access schemes, such as TDMA and FDMA as can be typically found in cellular networks.
A channel, in this context, refers to a system resource allocated to users or terminals, typically mobile users and wireless terminals, enabling establishing communication with the network or other users. Based on the channel attributes, terminal specifications, geographic distribution of users, nature of traffic, regulation, quality of service, and other factors, a specific multiple access scheme is defined.
In wireless networks and channels, the most important resource to consider for multiple access is the bandwidth allocated to that channel. As typically a limited amount of bandwidth is allocated to a wireless channel, a wireless system is required to accommodate as many users as possible by effectively sharing the limited bandwidth. Therefore, for wireless communications, the term multiple access could be defined as a means of allowing multiple users to simultaneously share the finite bandwidth with least possible degradation in the performance of the system.
The Time Division Multiple Access (TDMA) scheme provides time-slots to different data-streams (or transmitters), typically in a cyclically repetitive frame structure. For example, node 1 may use time slot 1, node 2 time slot 2, etc. until the last transmitter. Then it starts all over again, in a repetitive pattern. An advanced form is Dynamic TDMA (DTDMA), where the allocation of time slots to transmitters may change in real time. Packet Mode Multiple Access is a variation of TDMA, which is typically based on a random or pseudo-random access in time, preferably accounting for transmission collisions.
The Frequency Division Multiple Access (FDMA) scheme provides frequency bands to different data-streams, possibly on a dynamic basis. FDMA may allocate specific frequency bands to specific transmitters, for an entire session, or dynamically change that allocation, as done in Frequency Hopping (FH). An advanced form of FDMA is the Orthogonal Frequency Division Multiple Access (OFDMA) scheme, where each transmitter may use several sub-carriers, making it possible to provide different quality of service (different data rates) to different users. The assignment of sub-carriers to users may be changed dynamically, based on current radio channel conditions and traffic.
The Code Division Multiple Access (CDMA) scheme, also known as Spread Spectrum Multiple Access (SSMA), is based on spread spectrum, meaning that a wider radio spectrum is used than the data rate of each of the transferred bit streams, and several message signals are transferred simultaneously over the same carrier frequency, utilizing different spreading codes. The wide bandwidth makes it possible to send with very poor signal-to-noise ratio conditions.
One form is direct sequence spread spectrum (DS-CDMA), where each symbol (e.g. bit) is represented by a long code sequence of pulses, called chips. The sequence is the spreading code, and each message or transmitter uses a different spreading code, as done for example by different satellites broadcasting in a navigation system such as the GPS. Another form is frequency-hopping (FH-CDMA), where the channel frequency is changing rapidly according to a sequence that constitutes the spreading code.
The Space Division Multiple Access (SDMA) scheme is based on directional antennas or phased array techniques, enabling to focus the transmitting signal power in a narrow direction, or likewise limit the receiving noise to small angles. Examples of SDMA are sector antennas at cellular base stations, typically covering 120 degrees in azimuth.
Multiple access methods typically require careful synchronization among end users, either in time or frequency or code or orientation of antennas, particularly when the related channel resources (such as time-slots, frequency-bands, spreading-codes) are dynamically allocated. So basically, a specific channel access parameter such as time slot or center frequency, assigned to a specific user or transmission, reflects the need not to be in conflict with other users or transmissions, so obviously should be administered precisely.
It should also be noted that a multiple access method is typically (and not surprisingly) a method for efficiently accessing a communications channel or network, and not a method to communicate payload or application data. A system may sometimes use the multiple access method to communicate system data, such as the terminal ID, particularly when a specific multiple access parameter directly identifies a specific terminal (e.g.—terminal #1 is assigned with time-slot #1 or frequency-band #1), but typically not further. Payload data, application data or user defined data, is traditionally communicated by modulation of a carrier, typically an RF carrier in radio communications, after the channel is acquired.
Many modulation methods are known and practiced in the art, for analog information, such as: Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation (PM); and for digital information, such as: Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift Keying (PSK), and Minimal Shift Keying (MSK); and spread spectrum methods such as: Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), Orthogonal Frequency Division Multiplexing (OFDM).
Yet sometimes, even considering such efficient modulation schemes, there is a need to communicate more data, and increase the system throughput.
An example of such a need is related to the satellite Search and Rescue (SAR) system known as Cospas-Sarsat. Though the present invention is not limited to this specific system, Cospas-Sarsat (C/S) is a good example to clarify the present art, as well as the present invention, so it is specifically referred hereto. A detailed description of the system can be found in—www.cospas-sarsat.org.
Cospas-Sarsat is a satellite communication system to assist SAR of people in distress, all over the world and at anytime. The system was launched in 1982 by the USA, Canada, France and the Soviet Union (now Russia) and since then, it has been used for thousands of SAR events and has been instrumental in the rescue of over 30,000 lives worldwide. The goal of the system is to detect and locate signals transmitted from distress radio beacons and forward that data to ground stations, in order to support organizations responsibility for SAR operations. The system uses spacecraft—Low Earth Orbit (LEO) and Geostationary (GEO) satellites and in the future also Medium Earth Orbit (MEO) satellites, to detect and relay to ground stations signals transmitted from radio beacons in the 406 MHz band. When active, a C/S beacon repeatedly transmits a signal per 50 seconds, each for about 0.5 second. The position of the beacon is determined either by the Doppler shift of the received beacon signal or by position coordinates encoded in the signal, provided by a built-in Global Navigation Satellite System (GNSS) receiver. When the MEOSAR (segment of C/S SAR system related to the MEO satellites) will be fully operable, it could enable locating the beacon by trilateration, as the beacon signal will be (almost) simultaneously detected by at least three MEO satellites.
It should be noted that GNSS is usually a general term, as well as GPS (Global Positioning System) and SPS (Satellite Positioning System) and SNS (Satellite Navigation System); these acronyms may generalize particular systems such as the USA GPS or the Russian GLONASS or the European GALILEO. In the scope of the present invention, unless referring to a specific system, the terms GNSS and GPS usually refer to any satellite navigation system.
Several types of Cospas-Sarsat beacons are defined, mainly differing by mechanical structure or activation method, according to their main use: a) Emergency Position Indicating Radio Beacon (EPIRB) for marine use; b) Emergency Locator Transmitter (ELT) for aviation use; and c) Personal Locator Beacon (PLB) for personal and/or terrestrial use. For the purpose of the present invention, the terms EPIRB, PLB and sometimes also ELT are alternatively used, and unless indicated otherwise, relate to any type of position indicating radio beacon, and vice versa.
Present Cospas-Sarsat beacons optionally comprise a built-in GNSS receiver, configured to determine self position, in terms of geographical coordinates. These coordinates are encoded in the message communicated to SAR centers. Present Cospas-Sarsat standards define several message formats, in which said position can be encoded. Obviously, the number of bits allocated to this encoded position defines the resolution of this position reported to SAR centers. Presently, this resolution is limited, in the best case, to four seconds of geographical latitude or longitude, i.e. 4/3600 of an angle. Resolution of 4 seconds is equivalent, in worst case, to 4/60 of a nautical mile, i.e. approximately 125 meters.
For several scenarios, e.g. searching for a person at sea on a stormy dark night, an error of 125 m in determining the position might be critical. Yet, if six more bits could be communicated to augment the coarse latitude plus coarse longitude encoded according to the standard, the location uncertainty could shrink to 125/8, i.e. approximately 16 meters. At this distance, a person may be significantly better heard and seen even on a stormy night.
The present inventor already referred to such a scenario and to the more general problem, in U.S. Pat. No. 7,693,216—Modulating transmission timing for data communications. Katz disclosed a method for communicating data from a transmitter to a receiver, in a periodical burst transmission regime, by modulating the transmission period.
Though that method could be useful in many cases, it might present a problem in other cases, where it is important to distinguish between a change in channel access parameter (e.g. transmission time) due to the standard access scheme or due to additional information communicated thereby.
It is then an object of the present invention to use present communication schemes, typically designed for other purposes, in order to communicate data, either system data or application data, from a transmitter to a receiver, without significantly downgrade that scheme related to its original purpose.
It is also an object of the present invention to use a channel access scheme in order to communicate data from a transmitter to a receiver, without violating the related channel access standards.
It is still an object of the present invention to communicate data during channel accessing, without significantly disturbing the channel access process itself.
It is another object of the present invention to convey plain information from a transmitter to a receiver, during channel access, using at least one of the following multiple access schemes: Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), and Space Division Multiple Access (SDMA).
It is additionally an object of the present invention to communicate data during the channel access phase, by modulating at least one of the following channel access parameters: transmission time, transmission frequency, spreading code, radiation directivity, radiation polarization.
It is yet an object of the present invention to enable a radio beacon configured to periodically transmit short signals, to communicate additional data, without modifying the present signal format.
It is further an object of the present invention to enable a radio beacon configured to periodically transmit location reports, to communicate additional data upgrading the location report resolution, without modifying the present signal format.
It is yet another object of the present invention to enable a receiver decoding data communicated during channel access.
It is yet also an object of the present invention to provide a method to communicate data during channel access, such that can be easily implemented without a significant, if any, hardware change.
It is still another object of the present invention to employ GNSS timing and position information, in the process of conveying data through modulation of channel access parameters.
Other objects and advantages of the invention will become apparent as the description proceeds.